1. Field of the Invention
This invention relates generally to oxygen generating compositions, and more particularly concerns improved low temperature sensitivity oxygen generating compositions including iron, nickel, or cobalt powder as a fuel, a transition metal oxide as a catalyst, and a sodium silicate as a reaction rate and rheology modifier and chlorine suppresser.
2. Description of Related Art
Chemical oxygen generators are typically used in situations requiring emergency supplemental oxygen, such as in aviation, in submarines, diving and mountain climbing, for example, and in other similar settings where it is useful to furnish a convenient reliable supply of oxygen gas of breathable quality. Oxygen for such purposes must be of suitably high purity. For example, the requirements of SAE Aerospace Standard AS801OC are frequently applicable to oxygen utilized in aviation applications. Chemical oxygen generating compositions based upon the decomposition of alkali metal chlorates or perchlorates have long been used as an emergency source of breathable oxygen in passenger aircraft, for example. Oxygen generating compositions utilizing alkali metal chlorates or perchlorates are disclosed in U.S. Pat. Nos. 5,198,147; 5,279,761; and 5,298,187; each of which are incorporated herein by reference.
Oxygen generating compositions are commonly required to function within a wide range of environmental temperatures, as low as -30.degree. and as high as 60.degree. C., for example. Since the rate of decomposition of sodium chlorate is temperature dependent, an excess weight of the chemical oxygen generating composition is commonly used in order to insure that a sodium chlorate based composition will meet minimum oxygen generating specifications at both low and high temperatures. It would be desirable to provide oxygen generating compositions that are less temperature sensitive and can provide a more uniform rate of oxygen generation over an operating range of temperatures.
A chemical oxygen genrating candle or core typically has several layers. When the oxygen generating reaction is initiated at one end of the core, the reaction front typically propagates along the longitudinal axis through the layers toward the other end of the core as oxygen is generated. Ideally, the reaction zone should move at a steady, repeatable rate governed by the amounts of fuel and catalyst in the layers along the length of the core. In practice, however, the behavior of the oxygen generating reaction can be far from this ideal.
When expended chemical core residues are visually examined, several conditions can be observed which indicate behavior that occurred during the oxygen generating reaction. When the oxygen generating reaction has evolved at a steady and smooth rate, the pores left in the residue are typically small and uniform. The presence of large cavities typically indicate the formation of very large bubbles associated with very large bursts of oxygen release. Such large bubbles tend to perturb heat transfer into other regions of the core, and can result in a large burst of oxygen release follow by a temporary sharp decline or dip in oxygen evolution.
Gross physical distortion in the shape of the residue, relative to the shape of the unreacted core, can be evidence of a very runny reaction zone that can result in possible mechanical failure of the core in the event of exposure of the core to severe vibration during operation of the oxygen generator. On the other hand, relatively uniform, laminar patterns of pores in the residue is suggestive of a well ordered reaction zone. The presence of irregular swirls in the residue can indicate that the reaction zone was severely disturbed and may have mechanically collapsed, which can also be correlated with an irregular flow of oxygen.
The various reaction behaviors that are observable in the residues of oxygen generation cores are related to the melt properties of the core. The reaction temperature can reach 500.degree. C. or higher inside an operating chemical oxygen generating core. Because sodium chlorate melts at about 265.degree. C., during operation of the oxygen generator, sodium chlorate can melt in an unconstrained manner and form puddles that can cause the core to collapse. Unconstrained melting, puddling, and collapsing of the core can result in a disorganized, irregular reaction front and an irregular oxygen generation rate, causing variation in performance from core to core, and causing the oxygen generation rate and the rate at which the reaction zone moves to be more temperature dependent. Melting of the oxygen generating core under such conditions can also make the core vulnerable to high intensity vibrations. The forces exerted by evolving gas during solid phase decomposition of the oxygen generating reaction mixture can also cause the partially decomposed or undecomposed portion of the core to crack, resulting in an erratic oxygen generation rate. This phenomenon is particularly likely at lower environmental temperatures. Since a minimum oxygen flow and a minimum duration are required at all operating temperatures, a heavier core is commonly needed to insure that the oxygen flow curve does not dip below the required minimum specification for operation under cold conditions, and that the duration is longer than the required minimum specification for hot conditions.
In addition, when chemical cores melt in an unconstrained way, the melted material can come in contact with the oxygen generator housing, resulting in hot spots on the generator wall, which can result in temperatures that exceed applicable performance specifications. The duration of oxygen generation can also be much shorter at higher temperatures due to a poorly organized reaction zone, which can have a larger reacting volume than expected. Unfortunately, in conventional oxygen generating candles, providing a suitable performance across the full range of environmental temperatures in which the oxygen generator is to be used typically is accomplished by increasing the core weight to offset temperature dependence, to insure that both required minimum flow rates in colder temperatures and minimum duration specifications at high temperatures are met. It would therefore be desirable to provide an alternative solution to these problems that does not require increasing the core weight.
Iron fueled oxygen generating compositions utilizing alkali metal chlorates or perchlorates as oxygen generating sources commonly include calcium hydroxide or lithium peroxide as rate modifiers. Calcium hydroxide and lithium peroxide are very strong inhibitors of the decomposition of sodium chlorate, so that only small amounts can be used, making it necessary to perform a prolonged mixing to uniformly distribute these minor ingredients in sodium chlorate.
Iron fueled oxygen generating compositions utilizing alkali metal chlorates or perchlorates as oxygen generating sources are known that can include sodium silicates, such as sodium metasilicate or sodium orthosilicate to smooth out the chlorate decomposition and to suppress free chlorine formation. However, when used in combination with iron powder as a fuel, commonly only about 1% loading of sodium silicates is needed for these purposes, and the loading of sodium silicates commonly does not exceed 5%.
It is desirable to provide oxygen generating cores that do not melt in an unconstrained manner to form puddles, and that retain their structural integrity and shape during operation of the oxygen generator, allowing reduction or elimination of preformed insulation layers that are commonly used to increase the mechanical integrity of the operating core. It would be desirable to provide oxygen generating compositions that have lower sensitivity to environmental temperatures, and that are structurally more robust to withstand high levels of vibration during operation. It would also be desirable to reduce the probability of a localized high temperature spot on the generator wall, to lower the maximum wall temperature during operation. It would further be desirable to provide oxygen generating compositions that produce smoother oxygen flow curves and have lower temperature sensitivity. The present invention meets these needs.